Reversible resistive memory using diodes formed in CMOS processes as program selectors

ABSTRACT

Junction diodes fabricated in standard CMOS logic processes can be used as program selectors for reversible resistive memory cells that can be programmed based on magnitude, duration, voltage-limit, or current-limit of a supply voltage or current. These cells are PCM, RRAM, CBRAM, or other memory cells that have a reversible resistive element coupled to a diode. The diode can be constructed by P+ and N+ active regions on an N well as the P and N terminals of the diode. The memory cells can be used to construct a two-dimensional memory array with the N terminals of the diodes in a row connected as a wordline and the reversible resistive elements in a column connected as a bitline. By applying a voltage or a current to a selected bitline and to a selected wordline to turn on the diode, a selected cell can be programmed into different states reversibly based on magnitude, duration, voltage-limit, or current-limit. The data in the reversible resistive memory can also be read by turning on a selected wordline to couple a selected bitline to a sense amplifier. The wordlines may have high-resistivity local wordlines coupled to low-resistive global wordlines through conductive contact(s) or via(s).

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority benefit of U.S. Provisional PatentApplication No. 61/375,653, filed on Aug. 20, 2010 and entitled “Circuitand System of Using Junction Diode As Program Selector for ResistiveDevices in CMOS Logic Processes,” which is hereby incorporated herein byreference; and U.S. Provisional Patent Application No. 61/375,660, filedon Aug. 20, 2010 and entitled “Circuit and System of Using PolysiliconDiode As Program Selector for Resistive Devices in CMOS LogicProcesses,” which is hereby incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to programmable memory devices, such asprogrammable resistive devices for use in memory arrays.

2. Description of the Related Art

A programmable resistive device is generally referred to a device'sresistance states that may change after means of programming. Resistancestates can also be determined by resistance values. For example, aresistive device can be a One-Time Programmable (OTP) device, such aselectrical fuse, and the programming means can apply a high voltage toinduce a high current to flow through the OTP element. When a highcurrent flows through an OTP element by turning on a program selector,the OTP element can be programmed, or burned into a high or lowresistance state (depending on either fuse or anti-fuse).

An electrical fuse is a common OTP which is a programmable resistivedevice that can be constructed from a segment of interconnect, such aspolysilicon, silicided polysilicon, silicide, metal, metal alloy, orsome combination thereof. The metal can be aluminum, copper, or othertransition metals. One of the most commonly used electrical fuses is aCMOS gate, fabricated in silicided polysilicon, used as interconnect.The electrical fuse can also be one or more contacts or vias instead ofa segment of interconnect. A high current may blow the contact(s) orvia(s) into a very high resistance state. The electrical fuse can be ananti-fuse, where a high voltage makes the resistance lower, instead ofhigher. The anti-fuse can consist of one or more contacts or vias withan insulator in between. The anti-fuse can also be a CMOS gate coupledto a CMOS body with a thin gate oxide as insulator.

The programmable resistive device can be a reversible resistive devicethat can be programmed into a digital logic value “0” or “1”repetitively and reversibly. The programmable resistive device can befabricated from phase change material, such as Germanium(Ge),Antimony(Sb), and Tellurium(Te) with composition Ge₂Sb₂Te₅ (GST-225) orGeSbTe-like materials including compositions of Indium (In), Tin (Sn),or Selenium (Se). The phase change material can be programmed into ahigh resistance amorphous state or a low resistance crystalline state byapplying a short and high voltage pulse or a long and low voltage pulse,respectively. The reversible resistive device can be a Resistive RAM(RRAM) with cells fabricated from metal oxides between electrodes, suchas Pt/NiO/Pt, TiN/TiOx/HfO2/TiN, TiN/ZnO/Pt. The resistance states canbe changed reversibly and determined by polarity, magnitude, duration,or voltage/current-limit of pulse(s) to generate or annihilateconductive filaments. Another programmable resistive device similar toRRAM is a Conductive Bridge RAM (CBRAM) that is based onelectro-chemical deposition and removal of metal ions in a thinsolid-state electrolyte film. The electrodes can be an oxidizable anodeand an inert cathode and the electrolyte can be Ag- or Cu-dopedchalcogenide glass such as GeSe or GeS, etc. The resistance states canbe changed reversibly and determined by polarity, magnitude, duration,or voltage/current-limit of pulse(s) to generate or annihilateconductive bridges. The programmable resistive device can be an MRAM(Magnetic RAM) with cells fabricated from magnetic multi-layer stacksthat construct a Magnetic Tunnel Junction (MTJ). In a Spin TransferTorque MRAM (STT-MRAM) the direction of currents applied to an MTJdetermines parallel or anti-parallel states, and hence low or highresistance states.

A conventional programmable resistive memory cell is shown in FIG. 1.The cell 10 consists of a resistive element 11 and an NMOS programselector 12. The resistive element 11 is coupled to the drain of theNMOS 12 at one end, and to a positive voltage V+ at the other end. Thegate of the NMOS 12 is coupled to a select signal (Sel), and the sourceis coupled to a negative voltage V−. When a high voltage is applied toV+ and a low voltage to V−, the resistive device 10 can be programmed byraising the select signal (Sel) to turn on the NMOS 12. One of the mostcommon resistive elements is a silicided polysilicon, the same materialand fabricated at the same time as a MOS gate. The size of the NMOS 12,as program selector, needs to be large enough to deliver the requiredprogram current for a few microseconds. The program current for asilicided polysilicon is normally between a few milliamps for a fusewith width of 40 nm to about 20 mA for a fuse with width about 0.6 um.As a result, the cell size of an electrical fuse using silicidedpolysilicon tends to be very large.

Another conventional programmable resistive device 20 for Phase ChangeMemory (PCM) is shown in FIG. 2( a). The PCM cell 20 has a phase changefilm 21 and a bipolar transistor 22 as program selector with P+ emitter23, N-base 27, and P-sub collector 25. The phase change film 21 iscoupled to the emitter 23 of the bipolar transistor 22 at one end, andto a positive voltage V+ at the other. The N-type base 27 of bipolartransistor 22 is coupled to a negative voltage V−. The collector 25 iscoupled to ground. By applying a proper voltage between V+ and V− for aproper duration of time, the phase change film 21 can be programmed intohigh or low resistance states, depending on voltage and duration.Conventionally, to program a phase-change memory to a high resistancestate (or reset state) requires about 3V for 50 ns and consumes about300 uA of current, or to program a phase-change memory to a lowresistance state (or set state) requires about 2V for 300 ns andconsumes about 100 uA of current.

FIG. 2( b) shows a cross section of a conventional bipolar transistor22. The bipolar transistor 22 includes a P+ active region 23, a shallowN well 24, an N+ active region 27, a P-type substrate 25, and a ShallowTrench Isolation (STI) 26 for device isolation. The P+ active region 23and N+ active region 27 couple to the N well 24 are the P and Nterminals of the emitter-base diode of the bipolar transistor 22, whilethe P-type substrate 25 is the collector of the bipolar transistor 22.This cell configuration requires an N well 24 be shallower than the STI26 to properly isolate cells from each other and needs 3-4 more maskingsteps over the standard CMOS logic processes which makes it more costlyto fabricate.

Another programmable resistive device 20′ for Phase Change Memory (PCM)is shown in FIG. 2( c). The PCM cell 20′ has a phase change film 21′ anda diode 22′. The phase change film 21′ is coupled between an anode ofthe diode 22′ and a positive voltage V+. A cathode of the diode 22′ iscoupled to a negative voltage V−. By applying a proper voltage betweenV+ and V− for a proper duration of time, the phase change film 21′ canbe programmed into high or low resistance states, depending on voltageand duration. As an example of use of a diode as program selector foreach PCM cell as shown in FIG. 2( c), see Kwang-Jin Lee et al., “A 90 nm1.8V 512 Mb Diode-Switch PRAM with 266 MB/s Read Throughput,”International Solid-State Circuit Conference, 2007, pp. 472-273. Thoughthis technology can reduce the PCM cell size to only 6.8 F² (F standsfor feature size), the diode requires very complicated process steps,such as Selective Epitaxial Growth (SEG), to fabricate, which would bevery costly for embedded PCM applications.

FIGS. 3( a) and 3(b) show several embodiments of an electrical fuseelement 80 and 84, respectively, fabricated from an interconnect. Theinterconnect serves as a particular type of resistive element. Theresistive element has three parts: anode, cathode, and body. The anodeand cathode provide contacts for the resistive element to be connectedto other parts of circuits so that a current can flow from the anode tocathode through the body. The body width determines the current densityand hence the electro-migration threshold for a program current. FIG. 3(a) shows a conventional electrical fuse element 80 with an anode 81, acathode 82, and a body 83. This embodiment has a large symmetrical anodeand cathode. FIG. 3( b) shows another conventional electrical fuseelement 84 with an anode 85, a cathode 86, and a body 87. Thisembodiment has an asymmetrical shape with a large anode and a smallcathode to enhance the electro-migration effect based on polarity andreservoir effects. The polarity effect means that the electro-migrationalways starts from the cathode. The reservoir effect means that asmaller cathode makes electro-migration easier because the smaller areahas lesser ions to replenish voids when the electro-migration occurs.The fuse elements 80, 84 in FIGS. 3( a) and 3(b) are relatively largestructures which makes them unsuitable for some applications.

FIGS. 4( a) and 4(b) show programming a conventional MRAM cell 210 intoparallel (or state 0) and anti-parallel (or state 1) by currentdirections. The MRAM cell 210 consists of a Magnetic Tunnel Junction(MTJ) 211 and an NMOS program selector 218. The MTJ 211 has multiplelayers of ferromagnetic or anti-ferromagnetic stacks with metal oxide,such as Al₂O₃ or MgO, as an insulator in between. The MTJ 211 includes afree layer stack 212 on top and a fixed layer stack 213 underneath. Byapplying a proper current to the MTJ 211 with the program selector CMOS218 turned on, the free layer stack 212 can be aligned into parallel oranti-parallel to the fixed layer stack 213 depending on the currentflowing into or out of the fixed layer stack 213, respectively. Thus,the magnetic states can be programmed and the resultant states can bedetermined by resistance values, lower resistance for parallel andhigher resistance for anti-parallel states. The resistances in state 0or 1 are about 5KΩ or 10KΩ, respectively, and the program currents areabout +/−100-200 μA. One example of programming an MRAM cell isdescribed in T. Kawahara, “2 Mb Spin-Transfer Torque RAM with Bit-by-BitBidirectional Current Write and Parallelizing-Direction Current Read,”International Solid-State Circuit Conference, 2007, pp. 480-481.

SUMMARY OF THE INVENTION

Embodiments of programmable resistive device cells using junction diodesas program selectors are disclosed. The programmable resistive devicescan be fabricated using standard CMOS logic processes to reduce cellsize and cost.

In one embodiment, a programmable resistive device and memory can useP+/N well diodes as program selectors, where the P and N terminals ofthe diode are P+ and N+ active regions residing in an N well. The sameP+ and N+ active regions are used to create sources or drains of PMOSand NMOS devices, respectively. Advantageously, the same N well can beused to house PMOS in standard CMOS logic processes. By using P+/N welldiodes in standard CMOS processes, a small cell size can be achieved,without incurring any special processing or masks. Thus, costs can bereduced substantially for variously applications, such as embeddedapplications.

The invention can be implemented in numerous ways, including as amethod, system, device, or apparatus (including graphical user interfaceand computer readable medium). Several embodiments of the invention arediscussed below.

As a reversible resistive memory, one embodiment can, for example,include at least a plurality of reversible resistive cells. Each of thereversible resistive cells can include a reversible resistive film and adiode as program selector. The diode can have at least a first activeregion with a first type of dopant for a first terminal of the diode,and a second active region with a second type of dopant for a secondterminal of the diode, and both active regions being fabricated fromsources or drains of CMOS devices and residing in a common CMOS well.The first terminal of the diode can be coupled to a first terminal ofthe reversible resistive film, and the reversible resistive film can beprogrammed reversibly by conducting a current flowing through thereversible resistive film, wherein the current is high and/or shortduration for programming one state and low and/or long duration forprogramming another state. The embodiment of the reversible resistivememory can further include: a plurality of local wordlines, each coupledto a plurality of the reversible resistive cells via the second terminalof the diodes and having a first resistivity; a plurality of globalwordlines, each coupled to at least one of the local wordlines andhaving a second resistivity; and a plurality of bitlines, each coupledto a plurality of the reversible resistive cells via the second terminalof reversible resistive film.

As an electronics system, one embodiment can, for example, include atleast a processor; and a reversible resistive memory operativelyconnected to the processor. The reversible resistive memory can includeat least a plurality of reversible resistive cells. Each of thereversible resistive cells can include a reversible resistive film and adiode as program selector. The diode can have at least a first activeregion with a first type of dopant for a first terminal of the diode,and a second active region with a second type of dopant for a secondterminal of the diode, and both active regions being fabricated fromsources or drains of CMOS devices and residing in a common CMOS well.The first terminal of the diode can be coupled to a first terminal ofthe reversible resistive film. The reversible resistive film can beprogrammable by conducting a current flowing through the reversibleresistive film and the program selector, wherein the current is highand/or short duration for one state and low and/or long duration foranother state. The reversible resistive cells can also include at least:a plurality of local wordlines, each coupled to a plurality of thereversible resistive cells via the second terminal of the diodes andhaving a first resistivity; a plurality of global wordlines, eachcoupled to at least one of the local wordlines and having a secondresistivity; and a plurality of bitlines, each coupled to a plurality ofthe reversible resistive cells via the second terminal of reversibleresistive film.

As a method for programming a reversible resistive memory, oneembodiment can, for example, include at least providing a plurality ofreversible resistive cells, each including an reversible resistive filmand a diode The diode can serve as a program selector and can have atleast a first active region with a first type of dopant for a firstterminal of the diode, and having a second active region with a secondtype of dopant for a second terminal of the diode. Both active regionscan be fabricated from sources or drains of CMOS devices and can residein a common CMOS well. The first terminal of the diode can be coupled toa first terminal of the reversible resistive film, the reversibleresistive film can be programmable reversibly by conducting a currentflowing through the reversible resistive film. The embodiment of themethod can also include at least: providing a plurality of localwordlines, each coupled to a plurality of the reversible resistive cellsvia the second terminal of the diodes and having a first resistivity;providing a plurality of global wordlines, each coupled to at least oneof the local wordlines and having a second resistivity; providing aplurality of bitlines, each coupled to a plurality of the reversibleresistive cells via a second terminal of reversible resistive film; andprogramming at least one of the reversible resistive cells by applyingvoltages to a selected one of the global wordlines and to a selected oneof the bitlines, and generating a high and/or short-duration current toreversibly program into one state and a low and/or long-duration currentto reversibly program into another state.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be readily understood by the followingdetailed descriptions in conjunction with the accompanying drawings,wherein like reference numerals designate like structural elements, andin which:

FIG. 1 shows a conventional programmable resistive memory cell.

FIG. 2( a) shows another conventional programmable resistive device forPhase Change Memory (PCM) using bipolar transistor as program selector.

FIG. 2( b) shows a cross section of a conventional Phase Change Memory(PCM) using bipolar transistor as program selector.

FIG. 2( c) shows another conventional Phase Change Memory (PCM) cellusing diode as program selector.

FIGS. 3( a) and 3(b) show several embodiments of an electrical fuseelement, respectively, fabricated from an interconnect.

FIGS. 4( a) and 4(b) show programming a conventional MRAM cell intoparallel (or state 0) and anti-parallel (or state 1) by currentdirections.

FIG. 5( a) shows a block diagram of a memory cell using a junction diodeaccording to the invention.

FIG. 5( b) shows a cross section of a junction diode as program selectorwith STI isolation according to one embodiment.

FIG. 5( c) shows a cross section of a junction diode as program selectorwith CMOS gate isolation according to one embodiment.

FIG. 5( d) shows a cross section of a junction diode as program selectorwith SBL isolation according to one embodiment.

FIG. 6( a) shows a cross section of a junction diode as program selectorwith dummy CMOS gate isolation in SOI technologies according to oneembodiment.

FIG. 6( b) shows a cross section of a junction diode as program selectorwith dummy CMOS gate isolation in FINFET technologies according to oneembodiment.

FIG. 7( a) shows an electrical fuse element according to one embodiment.

FIG. 7( b) shows a top view of an electrical fuse coupled to a junctiondiode with STI isolation in four sides.

FIG. 7( c) shows a top view of an electrical fuse coupled to a junctiondiode with STI isolation in two sides and dummy CMOS isolation inanother two sides.

FIG. 7( d) shows a top view of an electrical fuse coupled to a junctiondiode with dummy CMOS isolation in four sides.

FIG. 7( e) shows a top view of an electrical fuse coupled to a junctiondiode with Silicide Block Layer isolation in four sides.

FIG. 7( f) shows an abutted contact coupled between a resistive element,P terminal of a junction diode, and metal in a single contact.

FIG. 8( a) shows a top view of a metal fuse coupled to a junction diodewith dummy CMOS gate isolation.

FIG. 8( b) shows a top view of a metal fuse coupled to a junction diodewith 4 cells sharing one N well contact in each side.

FIG. 8( c) shows a top view of a via1 fuse coupled to a junction diodewith 4 cells sharing one N well contact in each side.

FIG. 8( d) shows a top view of a two-dimensional array of via1 fusesusing P+/N well diodes.

FIG. 9( a) shows a cross section of a programmable resistive device cellusing phase-change material as a resistive element, with buffer metalsand a P+/N well junction diode, according to one embodiment.

FIG. 9( b) shows a top view of a PCM cell using a P+/N well junctiondiode as program selector in accordance with one embodiment.

FIG. 10 shows one embodiment of an MRAM cell using diodes as programselectors in accordance with one embodiment.

FIG. 11( a) shows a top view of an MRAM cell with an MTJ as a resistiveelement and with P+/N well diodes as program selectors in standard CMOSprocesses in accordance with one embodiment.

FIG. 11( b) shows another top view of an MRAM cell with an MTJ as aresistive element and with P+/N well diodes as program selectors in ashallow well CMOS process in accordance with another embodiment.

FIG. 12( a) shows one embodiment of a three-terminal 2×2 MRAM cell arrayusing junction diodes as program selectors and the condition to programthe upper-right cell into 1 in accordance with one embodiment.

FIG. 12( b) shows alternative conditions to program the upper-right cellinto 1 in a 2×2 MRAM array in accordance with one embodiment.

FIG. 13( a) shows one embodiment of a three-terminal 2×2 MRAM cell arrayusing junction diodes as program selectors and the condition to programthe upper-right cell into 0 in accordance with one embodiment.

FIG. 13( b) shows alternative conditions to program the upper-right cellinto 0 in a 2×2 MRAM array in accordance with one embodiment.

FIGS. 14( a) and 14(b) show one embodiment of programming 1 and 0 intothe upper-right cell, respectively, in a two-terminal 2×2 MRAM cellarray in accordance with one embodiment.

FIG. 15 shows a portion of a programmable resistive memory constructedby an array of n-row by (m+1)-column non-MRAM cells and n wordlinedrivers in accordance with one embodiment.

FIG. 16( a) shows a portion of a programmable resistive memoryconstructed by an array of 3-terminal MRAM cells according to oneembodiment.

FIG. 16( b) shows another embodiment of constructing a portion of MRAMmemory with 2-terminal MRAM cells.

FIGS. 17( a), 17(b), and 17(c) show three other embodiments ofconstructing reference cells for differential sensing.

FIG. 18( a) shows a schematic of a wordline driver circuit according toone embodiment.

FIG. 18( b) shows a schematic of a bitline circuit according to oneembodiment.

FIG. 18( c) shows a portion of memory with an internal power supply VDDPcoupled to an external supply VDDPP and a core logic supply VDD throughpower selectors.

FIG. 19( a) shows one embodiment of a schematic of a pre-amplifieraccording to one embodiment.

FIG. 19( b) shows one embodiment of a schematic of an amplifieraccording to one embodiment.

FIG. 19( c) shows a timing diagram of the pre-amplifier and theamplifier in FIGS. 19( a) and 19(b), respectively.

FIG. 20( a) shows another embodiment of a pre-amplifier, similar to thepre-amplifier in FIG. 18( a).

FIG. 20( b) shows level shifters according to one embodiment.

FIG. 20( c) shows another embodiment of an amplifier with current-mirrorloads.

FIG. 21( a) depicts a method of programming a programmable resistivememory in a flow chart according to one embodiment.

FIG. 21( b) depicts a method of reading a programmable resistive memoryin a flow chart according to one embodiment.

FIG. 22 shows a processor system according to one embodiment.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments disclosed herein use a P+/N well junction diode as programselector for a programmable resistive device. The diode can comprise P+and N+ active regions on an N well. Since the P+ and N+ active regionsand N well are readily available in standard CMOS logic processes, thesedevices can be formed in an efficient and cost effective manner. Thereare no additional masks or process steps to save costs. The programmableresistive device can also be included within an electronic system.

FIG. 5( a) shows a block diagram of a memory cell 30 using a junctiondiode according to one embodiment. In particular, the memory cell 30includes a resistive element 30 a and a junction diode 30 b. Theresistive element 30 a can be coupled between an anode of the junctiondiode 30 b and a positive voltage V+. A cathode of the junction diode 30b can be coupled to a negative voltage V−. In one implementation, thememory cell 30 can be a fuse cell with the resistive element 30 aoperating as an electrical fuse. The junction diode 30 b can serve as aprogram selector. The junction diode can be constructed from a P+/N wellin standard CMOS processes using a P-type substrate. The P+ and N+active regions serve as the anode and cathode of the diode are thesources or drains of CMOS devices. The N well is a CMOS well to housePMOS devices. Alternatively, the junction diode can be constructed fromN+/P well in triple-well or CMOS processes using an N-type substrate.The coupling of the resistive element 30 a and the junction diode 30 bbetween the supply voltages V+ and V− can be interchanged. By applying aproper voltage between V+ and V− for a proper duration of time, theresistive element 30 a can be programmed into high or low resistancestates, depending on voltage and duration, thereby programming thememory cell 30 to store a data value (e.g., bit of data). The P+ and N+active regions of the diode can be isolated by using a dummy CMOS gate,Shallow Trench Isolation (STI) or Local Oxidation (LOCOS), or SilicideBlock Layer (SBL).

Electrical fuse cell can be used as an example to illustrate the keyconcepts according to one embodiment. FIG. 5( b) shows a cross sectionof a diode 32 using a P+/N well diode as program selector with ShallowTrench Isolation (STI) isolation in a programmable resistive device. P+active region 33 and N+ active region 37, constituting the P and Nterminals of the diode 32 respectively, are sources or drains of PMOSand NMOS in standard CMOS logic processes. The N+ active region 37 iscoupled to an N well 34, which houses PMOS in standard CMOS logicprocesses. P-substrate 35 is a P-type silicon substrate. STI 36 isolatesactive regions for different devices. A resistive element (not shown inFIG. 5( b)), such as electrical fuse, can be coupled to the P+ region 33at one end and to a high voltage supply V+ at the other end. To programthis programmable resistive device, a high voltage is applied to V+, anda low voltage or ground is applied to the N+ region 37. As a result, ahigh current flows through the fuse element and the diode 32 to programthe resistive device accordingly.

FIG. 5( c) shows a cross section of another embodiment of a junctiondiode 32′ as program selector with dummy CMOS gate isolation. ShallowTrench Isolation (STI) 36′ provides isolation among active regions. Anactive region 31′ is defined between STI 36′, where the N+ and P+ activeregions 37′ and 33′ are further defined by a combination of a dummy CMOSgate 39′, P+ implant layer 38′, and N+ implant (the complement of the P+implant 38′), respectively, to constitute the N and P terminals of thediode 32′. The diode 32′ is fabricated as a PMOS-like device with 37′,39′, 33′, and 34′ as source, gate, drain, and N well, except that thesource 37′ is covered by an N+ implant, rather than a P+ implant 38′.The dummy MOS gate 39′, preferably biased at a fixed voltage, onlyserves for isolation between P+ active region 33′ and N+ active region37′ during fabrication. The N+ active 37′ is coupled to an N well 34′,which houses PMOS in standard CMOS logic processes. P-substrate 35′ is aP-type silicon substrate. A resistive element (not shown in FIG. 5( c)),such as electrical fuse, can be coupled to the P+ region 33′ at one endand to a high voltage supply V+ at the other end. To program thisprogrammable resistive device, a high voltage is applied to V+, and alow voltage or ground is applied to the N+ active region 37′. As aresult, a high current flows through the fuse element and the diode 32′to program the resistive device accordingly. This embodiment isdesirable for isolation for small size and low resistance.

FIG. 5( d) shows a cross section of another embodiment of a junctiondiode 32″ as program selector with Silicide Block Layer (SBL) isolation.FIG. 5( d) is similar to 5(c), except that the dummy CMOS gate 39′ inFIG. 5( c) is replaced by SBL 39″ in FIG. 5( d) to block a silicidegrown on the top of active region 31″. Without a dummy MOS gate or aSBL, the N+ and P+ active regions would be undesirably electricallyshorted by a silicide on the surface of the active region 31″.

FIG. 6( a) shows a cross section of another embodiment of a junctiondiode 32″ as a program selector in Silicon-On-Insulator (SOI)technologies. In SOI technologies, the substrate 35″ is an insulatorsuch as SiO₂ or similar material with a thin layer of silicon grown ontop. All NMOS and PMOS are in silicon wells isolated by SiO₂ or similarmaterial to each other and to the substrate 35″. One-piece active region31″ is divided into an N+ active region 37″, P+ active region 33″, andbody 34″ by a combination of a dummy CMOS gate 39″, P+ implant 38″, andN+ implant (the complement of P+ implant 38″). Consequently, the N+active region 37″ and P+ active region 33″ constitute the N and Pterminals of the junction diode 32″. The N+ active region 37″ and P+active region 33″ can be the same as sources or drains of NMOS and PMOSdevices, respectively, in standard CMOS processes. Similarly, the dummyCMOS gate 39″ can be the same CMOS gate fabricated in standard CMOSprocesses. The dummy MOS gate 39″, which can be biased at a fixedvoltage, only serves for isolation between P+ active region 33″ and N+active region 37″ during fabrication. The N+ active region 37″ can becoupled to a low voltage supply V− and to an N well 34″ that houses PMOSin standard CMOS SOI processes. A resistive element (not shown in FIG.6( a)), such as an electrical fuse, can be coupled to the P+ activeregion 33″ at one end and to a high voltage supply V+ at the other end.To program the electrical fuse cell, a high and low voltages are appliedto V+ and V−, respectively, to conduct a high current flowing throughthe fuse element and the junction diode 32″ to program the resistivedevice accordingly. Other embodiments of isolations in CMOS bulktechnologies, such as STI, dummy MOS gate, or SBL in one to four (1-4)or any sides, can be readily applied to CMOS SOI technologiesaccordingly.

FIG. 6( b) shows a cross section of another embodiment of a junctiondiode 45 as a program selector in FinFET technologies. FinFET refers toa fin-based, multigate transistor. FinFET technologies are similar tothe conventional CMOS except that thin and tall silicon islands can beraised above the silicon substrate to serve as the bulks of CMOSdevices. The bulks are divided into source, drain, and channel regionsby polysilicon or non-aluminum metal gates like in the conventionalCMOS. The primary difference is that the MOS devices are raised abovethe substrate so that channel widths are the height of the islands,though the direction of current flow is still in parallel to thesurface. In an example of FinFET technology shown in FIG. 6( b), thesilicon substrate 35 is an epitaxial layer built on top of an insulatorlike SOI or other high resistivity silicon substrate. The siliconsubstrate 35 can then be etched into several tall rectangular islands31-1, 31-2, and 31-3. With proper gate oxide grown, the islands 31-1,31-2, and 31-3 can be patterned with MOS gates 39-1, 39-2, and 39-3,respectively, to cover both sides of raised islands 31-1, 31-2, and 31-3and to define source and drain regions. The source and drain regionsformed at the islands 31-1, 31-2, and 31-3 are then filled with silicon,such as fill 40-1 and 40-2, so that the combined source or drain areasare large enough to allow contacts. The fill 40-1 and 40-2 areas in FIG.6( b) are for illustrative purpose to reveal the cross section and can,for example, be filled up to the surface of the islands 31-1, 31-2, and31-3. In this embodiment, active regions 33-1,2,3 and 37-1,2,3 arecovered by a P+ implant 38 and N+ implant (the complement of P+ implant38), respectively, rather than all covered by P+ implant 38 as PMOS inthe conventional FinFET, to constitute the P and N terminals of thejunction diode 45. The N+ active region 37-1,2,3 is coupled to a lowvoltage supply V−. A resistive element (not shown in FIG. 6( b)), suchas an electrical fuse, is coupled to the P+ active region 33-1,2,3 atone end and to a high voltage supply V+ at the other end. To program theelectrical fuse, high and low voltages are applied between V+ and V−,respectively, to conduct a high current flowing through the resistiveelement and the junction diode 45 to program the resistive deviceaccordingly. Other embodiments of isolations in CMOS bulk technologies,such as STI, dummy MOS gate or SBL, can be readily applied to FinFETtechnologies accordingly.

FIG. 7( a) shows a top view of an electrical fuse element 88 accordingto one embodiment. The electrical fuse element 88 can, for example, byused as the resistive element 31 a illustrated in FIG. 5( a). Theelectrical fuse element 88 includes an anode 89, a cathode 90, and abody 91. In this embodiment, the electrical fuse element 88 is a barshape with a small anode 89 and cathode 90 to reduce area. The anode 89and cathode 90 may protrude from the body 91 to make contacts. Thecontact number can be one (1) for both the anode 89 and the cathode 90so that the area can be very small. However, the contact area for anode89 is often made larger so that the anode 89 can resistelectro-migration more than the cathode 90. The fuse body 91 can haveabout 1-5 squares, namely, the length to width ratio is about 1-to-5, tooptimize cell area and program current. The fuse element 88 has a P+implant 92 covering half of the body 91 and the cathode 90, while an N+implant over the rest of area. This embodiment makes the fuse element 88behave like a reverse biased diode to increase resistance after beingprogrammed, when silicide on top is depleted by electro-migration, iondiffusion, silicide decomposition, and other effects.

The above scheme can be realized for those fuse elements consisting ofpolysilicon, silicided polysilicon, or other CMOS gate material so thatP+ and N+ implants can create a diode. For example, if a metal-gate CMOShas a sandwich structure of polysilicon between metal alloy layers, themetal alloy layers may be blocked by masks generated from layoutdatabase to create a diode in the fuse elements.

FIGS. 7( b), 7(c), 7(d), 7(e), and 7(f) show top views of P+/N welldiodes constructed with different embodiments of isolation and fuseelements. Without isolation, P+ and N+ active regions would be shortedtogether by silicide grown on top. The isolation can be provided by STI,dummy CMOS gate, SBL, or some combination thereof from one to four (1-4)or any sides. The P+ and N+ active regions that act as P and N terminalsof the diodes are sources or drains of CMOS devices. Both the P+ and N+active regions reside in an N well, which is the same N well that can beused to house PMOS in standard CMOS processes. The N+ active region ofthe diodes in multiple cells can be shared, though for simplicity FIGS.7( b)-7(f) show only one N+ active region for one P+ active region.

FIG. 7( b) shows a top view of one embodiment of a P+/N well diode 40 inan electrical fuse cell having active regions 43 and 44 with STI 49isolation in four sides. A fuse element 42 is coupled to the activeregion 43 through a metal 46. The active regions 43 and 44 are coveredby a P+ implant 47 and N+ implant (the complement of P+ implant 47),respectively, to constitute the P and N terminals of the diode 40. Theactive regions 43 and 44 of the diode 40 reside in an N well 45, thesame N well can be used to house PMOS in standard CMOS processes. Inthis embodiment, the P+ active region 43 and N+ active region 44 aresurrounded by an STI 49 in four (4) sides. Since the STI 49 is muchdeeper than either the N+ or P+ active region, the resistance of thediode 40 between the P+ active region 43 and N+ active region 44 ishigh.

FIG. 7( c) shows a top view of another embodiment of a P+/N well diode50 in an electrical fuse cell having active regions 53 and 54 with anSTI 59 isolation in two sides and a dummy MOS gate 58 in another twosides. One-piece active region 51 with two STI slots 59 in the right andleft is divided into a peripheral 54 and a central 53 regions by two MOSgates 58 on top and bottom. The central active region 53 is covered by aP+ implant 57, while the peripheral active region 54 is covered by an N+implant layer (the complement of the P+ implant), which constitute the Pand N terminals of the diode 50. The active region 51 resides in an Nwell 55, the same N well can be used to house PMOS in standard CMOSprocesses. A fuse element 52 is coupled to the P+ active region 53. Thedummy MOS gate 58 is preferably biased to a fixed voltage. In thisembodiment, the P+ active region 53 and N+ active region 54 aresurrounded by STI 59 in left and right sides and the dummy MOS gate 58on top and bottom. The isolation provided by the dummy MOS gate 58 canprovide lower resistance than the STI isolation, because the spacebetween the P+ active region 53 and N+ active region 54 may be narrowerand there is no oxide to block the current path underneath the siliconsurface.

FIG. 7( d) shows a top view of yet another embodiment of P+/N well diode60 in an electrical fuse cell with dummy MOS gate 68 providing isolationin four sides. One-piece active region 61 is divided into a centeractive region 63 and a peripheral active region 64 by a ring-shape MOSgate 68. The center active region 63 is covered by a P+ implant 67 andthe peripheral active region 64 is covered by an N+ implant (thecomplement of the P+ implant 67), respectively, to constitute the P andN terminals of the diode 60. The active region 61 resides in an N well,the same N well can be used to house PMOS in standard CMOS processes. Afuse element 62 is coupled to the P+ active region 63 through a metal66. The dummy MOS gate 68, which can be biased at a fixed voltage,provides isolation between P+ active region 63 and N+ active region 64regions on four sides. This embodiment offers low resistance between Pand N terminals of the diode 60.

FIG. 7( e) shows a top view of yet another embodiment of a P+/N welldiode 60′ in an electrical fuse cell having active regions 63′ and 64′with Silicide Block Layer (SBL) 68′ providing isolation in four sides.One-piece active region 61′ is divided into a center active region 63′and a peripheral active region 64′ by an SBL ring 68′. The center activeregion 63′ and the peripheral active region 64′ are covered by a P+implant 67′ and an N+ implant (the complement of P+ implant 67′),respectively, to constitute the P and N terminals of the diode 60′. Theboundaries between the P+ implant 67′ and N+ implants are about in themiddle of the SBL ring 68′. The active region 61′ resides in an N well65′. A fuse element 62′ is coupled to the P+ active region 63′ through ametal 66′. The SBL ring 68′ blocks silicide formation on the top of theactive regions between P+ active region 63′ and N+ active region 64′. Inthis embodiment, the P+ active region 63′ and N+ active region 64′ areisolated in four sides by P/N junctions. This embodiment has lowresistance between the P and N terminals of the diode 60′, though theSBL may be wider than a MOS gate. In another embodiment, there is aspace between the P+ implant 67′ and the N+ implant that is covered bythe SBL ring 68′.

FIG. 7( f) shows a top view of another embodiment of a P+/N well diode70 in an electrical fuse cell with an abutted contact. Active regions 73and 74, which are isolated by an STI 79, are covered by a P+ implant 77and an N+ implant (the complement of the P+ implant 77), respectively,to constitute the P and N terminals of the diode 70. Both of the activeregions 73 and 74 reside in an N well 75, the same N well can be used tohouse PMOS in standard CMOS processes. A fuse element 72 is coupled tothe P+ active region 73 through a metal 76 in a single contact 71. Thiscontact 71 is quite different from the contacts in FIGS. 7( b), (c),(d), and (e) where a contact can be used to connect a fuse element witha metal and then another contact is used to connect the metal with a P+active region. By connecting a fuse element directly to an active regionthrough a metal in a single contact, the cell area can be reducedsubstantially. This embodiment for a fuse element can be constructed bya CMOS gate, including polysilicon, silicided polysilicon, ornon-aluminum metal CMOS gate, that allows an abutted contact.

In general, a polysilicon or silicide polysilicon fuse is more commonlyused as an electrical fuse because of its lower program current thanmetal or contact/via fuses. However, a metal fuse has some advantagessuch as smaller size and wide resistance ratio after being programmed.Metal as a fuse element allows making contacts directly to a P+ activeregion thus eliminating one additional contact as compared to using apolysilicon fuse. In advanced CMOS technologies with feature size lessthan 65 nm, the program voltage for metal fuses can be lower than 3.3V,which makes metal fuse a viable solution.

FIG. 8( a) shows a top view of P+/N well diode 60″ having a metal1 fusewith dummy CMOS gate isolation. One-piece active region 61 is dividedinto a center active region 63 and a peripheral active region 64 by aring-shape MOS gate 68. The center active region 63 is covered by a P+implant 67 and the peripheral active region 64 is covered by an N+implant (the complement of the P+ implant 67), respectively, toconstitute the P and N terminals of the diode 60″. The active region 61resides in an N well 65, the same N well can be used to house PMOS instandard CMOS processes. A metal1 fuse element 62″ is coupled to the P+region 63 directly. The ring-shape MOS gate 68, which provides dummyCMOS gate isolation, can be biased at a fixed voltage, and can provideisolation between P+ active 63 and N+ active 64 regions in four sides.In one embodiment, the length to width ratio of a metal fuse is about1-5.

The size of the metal fuse cell in FIG. 8( a) can be further reduced, ifthe turn-on resistance of the diode is not crucial. FIG. 8( b) shows atop view of a row of metal fuse cells 60″′ having four metal fuse cellsthat share one N well contact in each side in accordance with oneembodiment. Metal1 fuse 69 has an anode 62′, a metal1 body 66′, and acathode coupled to an active region 64′ covered by a P+ implant 67′ thatacts as the P terminal of a diode. The active region 61′ resides in an Nwell 65′. Another active region 63′ covered by an N+ implant (complementof P+ implant 67′) acts as N terminal of the diode. Four diodes areisolated by STI 68′ and share one N+ active region 63′ each side. The N+active regions 63′ are connected by a metal2 running horizontally, andthe anode of the diode is connected by a metal3 running vertically. Ifmetal1 is intended to be programmed, other types of metals in theconduction path should be wider. Similarly, more contacts and viasshould be put in the conduction path to resist programming. It should benoted metal1 as a metal fuse in FIG. 8( b) is for illustrative purposes,those skilled in the art understand that the above description can beapplied to any metals, such as metal2, metal3, or metal4 in otherembodiments. Similarly, those skilled in the art understand that theisolation, metal scheme, and the number of cells sharing one N+ activemay vary in other embodiments.

Contact or via fuses may become more viable for advanced CMOStechnologies with feature size less than 65 nm, because smallcontact/via size makes program current rather low. FIG. 8( c) shows atop view of a row of four via1 fuse cells 70 sharing N-type wellcontacts 73 a and 73 b in accordance with one embodiment. Via1 fuse cell79 has a via1 79 a coupled to a metal1 76 and a metal2 72. Metal2 72 iscoupled to a metal3 through via2 89 running vertically as a bitline.Metal1 76 is coupled to an active region 74 covered by a P+ implant 77that acts as the P terminal of a diode 71. Active regions 73 a and 73 bcovered by an N+ implant (complement of P+ implant 77) serves as the Nterminal of the diode 71 in via1 fuse cell 79. Moreover, the activeregions 73 a and 73 b serve as the common N terminal of the diodes inthe four-fuse cell 70. They are further coupled to a metal4 runninghorizontally as a wordline. The active regions 74, 73 a, and 73 b residein the same N well 75. Four diodes in via1 fuse cells 70 have STI 78isolation between each other. If via1 is intended to be programmed, morecontacts and more other kinds of vias should be put in the conductionpath. And metals in the conduction path should be wider and containlarge contact/via enclosures to resist programming. Via1 as a via fusein FIG. 8( c) is for illustrative purpose, those skilled in the artunderstand that the above description can be applied to any kinds ofcontacts or vias, such as via2, via3, or via4, etc. Similarly, thoseskilled in the art understand that the isolation, metal scheme, and thenumber of cells sharing one N+ active may vary in other embodiments.

FIG. 8( d) shows a top view of an array of 4×5 via1 fuses with dummyCMOS gate isolation in accordance with one embodiment. The one-row viafuse shown in FIG. 8( c) can be extended into a two-dimensional array 90as shown in FIG. 8( d). The array 90 has four rows of active regions 91,each residing in a separate N well, and five columns of via fuse cells96, isolated by dummy CMOS gates 92 between active regions. Each viafuse cell 96 has one contact 99 on an active region covered by a P+implant 94 that acts as the P terminal of a diode, which is furthercoupled to a metal2 bitline running vertically. Active regions in twosides of the array 90 are covered by N+ implant 97 to serve as the Nterminals of the diodes in the same row, which is further coupled tometal3 as wordlines running horizontally. To program a via fuse, selectand apply voltages to the desired wordline and bitline to conduct acurrent from metal2 bitline, via1, metal1, contact, P+ active, N+active, to metal3 wordline. To ensure only via1 is programmed, metalscan be made wider and the numbers of other types of vias or contact canbe more than one. To simplify the drawing, metal1-via1-metal2 connectioncan be referred to FIG. 8( c) and, therefore, is not shown in each cellin FIG. 8( d). Those skilled in the art understand that various types ofcontact or vias can be used as resistive elements and the metal schemesmay change in other embodiments. Similarly, the number of cells in rowsand columns, the numbers of rows or columns in an array, and the numbersof cells between N+ active may vary in other embodiments.

FIG. 9( a) shows a cross section of a programmable resistive device cell40 using phase-change material as a resistive element 42, with buffermetals 41 and 43, and a P+/N well diode 32, according to one embodiment.The P+/N well diode 32 has a P+ active region 33 and N+ active region 37on an N well 34 as P and N terminals. The isolation between the P+active region 33 and N+ active region 37 is an STI 36. The P+ activeregion 33 of the diode 32 is coupled to a lower metal 41 as a bufferlayer through a contact plug 40-1. The lower metal 41 is then coupled toa thin film of phase change material 42 (e.g., GST film). An upper metal43 also couples to the thin film of the phase-change material 42 througha contact plug 40-2. The upper metal 43 is coupled to another metal 44to act as a bitline (BL) through a plug 40-3. The phase-change film 42can have a chemical composition of Germanium (Ge), Antimony (Sb), andTellurium (Te), such as Ge_(x)Sb_(y)Te_(z) (x, y and z are any arbitrarynumbers), or as one example Ge₂Sb₂Te₅ (GST-225). The GST film can bedoped with at least one or more of Indium (In), Tin (Sn), or Selenium(Se) to enhance performance. The phase-change cell structure can besubstantially planar, which means the phase-change film 42 has an areathat is larger than the film contact area coupled to the programselector, or the height from the surface of the silicon substrate to thephase-change film 42 is much smaller than the dimensions of the filmparallel to silicon substrate. In this embodiment, the active area ofphase-change film 42 is much larger than the contact area so that theprogramming characteristics can be more uniform and reproducible. Thephase-change film 42 is not a vertical structure and does not sit on topof a tall contact, which can be more suitable for embedded phase-changememory applications, especially when the diode 32 (i.e., junction diode)is used as program selector to make the cell size very small. For thoseskilled in the art understand that the structure and fabricationprocesses may vary and that the structures of phase-change film (e.g.,GST film) and buffer metals described above are for illustrativepurpose.

FIG. 9( b) shows a top view of a PCM cell using a junction diode asprogram selector having a cell boundary 80 in accordance with oneembodiment. The PCM cell has a P+/N well diode and a phase-changematerial 85, which can be a GST film. The P+/N well diode has activeregions 83 and 81 covered by a P+ implant 86 and an N+ implant(complement of P+ implant 86), respectively, to serve as the anode andcathode. Both active regions 81 and 83 reside on an N well 84, the sameN well can be used to house PMOS in standard CMOS processes. The anodeis coupled to the phase-change material 85 through a metal1 82. Thephase-change material 85 is further coupled to a metal3 bitline (BL) 88running vertically. The cathode of the P+/N well diode (i.e., activeregion 81) is connected by a metal2 wordline (WL) 87 runninghorizontally. By applying a proper voltage between the bitline 88 andthe wordline 87 for a suitable duration, the phase-change material 85can be programmed into a 0 or 1 state accordingly. Since programming thePCM cell is based on raising the temperature rather thanelectro-migration as with an electrical fuse, the phase-change film(e.g., GST film) can be symmetrical in area for both anode and cathode.Those skilled in the art understand that the phase-change film,structure, layout style, and metal schemes may vary in otherembodiments.

Programming a phase-change memory (PCM), such as a phase-change film,depends on the physical properties of the phase-change film, such asglass transition and melting temperatures. To reset, the phase-changefilm needs to be heated up beyond the melting temperature and thenquenched. To set, the phase-change film needs to be heated up betweenmelting and glass transition temperatures and then annealed. A typicalPCM film has glass transition temperature of about 200° C. and meltingtemperature of about 600° C. These temperatures determine the operationtemperature of a PCM memory because the resistance state may changeafter staying in a particular temperature for a long time. However, mostapplications require retaining data for 10 years for the operationtemperature from 0 to 85° C. or even from −40 to 125° C. To maintaincell stability over the device's lifetime and over such a widetemperature range, periodic reading and then writing back data into thesame cells can be performed. The refresh period can be quite long, suchas longer than a second (e.g., minutes, hours, days, weeks, or evenmonths). The refresh mechanism can be generated inside the memory ortriggered from outside the memory. The long refresh period to maintaincell stability can also be applied to other emerging memories such asRRAM, CBRAM, and MRAM, etc.

FIG. 10 shows one embodiment of an MRAM cell 310 using diodes 317 and318 as program selectors in accordance with one embodiment. The MRAMcell 310 in FIG. 10 is a three-terminal MRAM cell. The MRAM cell 310 hasan MTJ 311, including a free layer stack 312, a fixed layer stack 313,and a dielectric film in between, and the two diodes 317 and 318. Thefree layer stack 312 is coupled to a supply voltage V, and coupled tothe fixed layer stack 313 through a metal oxide such as Al₂O₃ or MgO.The diode 317 has the N terminal coupled to the fixed layer stack 313and the P terminal coupled to V+ for programming a 1. The diode 318 hasthe P terminal coupled to the fixed layer stack 313 and the N terminalcoupled to V− for programming a 0. If V+ voltage is higher than V, acurrent flows from V+ to V to program the MTJ 311 into state 1.Similarly, if V− voltage is lower than V, a current flows from V to V−to program the MTJ 311 into state 0. During programming, the other diodeis supposedly cutoff. For reading, V+ and V− can be both set to 0V andthe resistance between node V and V+/V− can be sensed to determinewhether the MTJ 311 is in state 0 or 1.

FIG. 11( a) shows a cross section of one embodiment of an MRAM cell 310with MTJ 311 and junction diodes 317 and 318 as program selectors inaccordance with one embodiment. MTJ 311 has a free layer stack 312 ontop and a fixed layer stack 313 underneath with a dielectric in betweento constitute a magnetic tunneling junction. Diode 317 is used toprogram 1 and diode 318 is used to program 0. Diodes 317 and 318 have P+and N+ active regions on N wells 321 and 320, respectively, the same Nwells to house PMOS in standard CMOS processes. Diode 317 has a P+active region 315 and N+ active region 314 to constitute the P and Nterminals of the program-1 diode 317. Similarly, diode 318 has a P+active 316 and N+ active 319 to constitute the P and N terminals of theprogram-0 diode 318. FIG. 11( a) shows STI 330 isolation for the P and Nterminals of diodes 317 and 318. For those skilled in the art understandthat different isolation schemes, such as dummy MOS gate or SBL, canalternatively be applied.

The free stacks 312 of the MTJ 311 can be coupled to a supply voltage V,while the N terminal of the diode 318 can be coupled to a supply voltageV− and the P terminal of the diode 317 can be coupled to another supplyvoltage V+. Programming a 1 in FIG. 11( a) can be achieved by applying ahigh voltage, i.e., 2V to V+ and V−, while keeping V at ground, or 0V.To program a 1, a current flows from diode 317 through the MTJ 311 whilethe diode 318 is cutoff. Similarly, programming a 0 can be achieved byapplying a high voltage to V, i.e., 2V, and keeping V+ and V− at ground.In this case. a current flows from MTJ 311 through diode 318 while thediode 317 is cutoff.

FIG. 11( b) shows a cross section of another embodiment of an MRAM cell310′ with MTJ 311′ and junction diodes 317′ and 318′ as programselectors in accordance with one embodiment. MTJ 311′ has a free layerstack 312′ on top and a fixed layer stack 313′ underneath with adielectric in between to constitute a magnetic tunneling junction. Diode317′ is used to program 1 and diode 318′ is used to program 0. Diodes317′ and 318′ have P+ and N+ active regions on N wells 321′ and 320′,respectively, which are fabricated by shallow N wells with additionalprocess steps. Though more process steps are needed, the cell size canbe smaller. Diode 317′ has P+ active region 315′ and N+ active region314′ to constitute the P and N terminals of the program-1 diode 317′.Similarly, diode 318′ has P+ active 316′ and N+ active 319′ toconstitute the P and N terminals of the program-0 diode 318′. STI 330′isolates different active regions.

The free stacks 312′ of the MTJ 311′ can be coupled to a supply voltageV, while the N terminal of the diode 318′ can be coupled to a supplyvoltage V− and the P terminal of the diode 317′ is coupled to anothersupply voltage V+. Programming a 1 in FIG. 11( b) can be achieved byapplying a high voltage, i.e., 2V to V+ and V−, while keeping V atground, or 0V. To program a 1, a current will flow from diode 317′through the MTJ 311′ while the diode 318′ is cutoff. Similarly,programming 0 can be achieved by applying a high voltage to V, i.e., 2V,and keeping V+ and V− at ground. In this case, a current will flow fromMTJ 311′ through diode 318′ while the diode 317′ is cutoff.

FIG. 12( a) shows one embodiment of a three-terminal 2×2 MRAM cell arrayusing junction diodes 317 and 318 as program selectors and the conditionto program 1 in a cell in accordance with one embodiment. Cells 310-00,310-01, 310-10, and 310-11 are organized as a two-dimensional array. Thecell 310-00 has a MTJ 311-00, a program-1 diode 317-00, and a program-0diode 318-00. The MTJ 311-00 is coupled to a supply voltage V at oneend, to the N terminal of the program-1 diode 317-00 and to the Pterminal of the program-0 diode 318-00 at the other end. The P terminalof the program-1 diode 317-00 is coupled to a supply voltage V+. The Nterminal of the program-0 diode 318-00 is coupled to another supplyvoltage V−. The other cells 310-01, 310-10, and 310-11 are similarlycoupled. The voltage Vs of the cells 310-00 and 310-10 in the samecolumns are connected to BL0. The voltage Vs of the cells 310-01 and310-11 in the same column are connected to BL1. The voltages V+ and V−of the cells 310-00 and 310-01 in the same row are connected to WL0P andWL0N, respectively. The voltages V+ and V− of the cells 310-10 and310-11 in the same row are connected to WL1P and WL1N, respectively. Toprogram a 1 into the cell 310-01, WL0P is set high and BL1 is set low,while setting the other BL and WLs at proper voltages as shown in FIG.12( a) to disable the other program-1 and program-0 diodes. The boldline in FIG. 12( a) shows the direction of current flow.

FIG. 12( b) shows alternative program-1 conditions for the cell 310-01in a 2×2 MRAM array in accordance with one embodiment. For example, toprogram a 1 into cell 310-01, set BL1 and WL0P to low and high,respectively. If BL0 is set to high in condition 1, the WL0N and WL1Ncan be either high or floating, and WL1P can be either low or floating.The high and low voltages of an MRAM in today's technologies are about2-3V for high voltage and 0 for low voltage, respectively. If BL0 isfloating in condition 2, WL0N and WL1N can be high, low, or floating,and WL1P can be either low or floating. In a practical implementation,the floating nodes are usually coupled to very weak devices to a fixedvoltage to prevent leakage. One embodiment of the program-1 condition isshown in FIG. 12( a) without any nodes floating.

FIG. 13( a) shows one embodiment of a three-terminal 2×2 MRAM cell arraywith MTJ 311 and junction diodes 317 and 318 as program selectors andthe condition to program 0 in a cell in accordance with one embodiment.The cells 310-00, 310-01, 310-10, and 310-11 are organized as atwo-dimensional array. The cell 310-00 has a MTJ 311-00, a program-1diode 317-00, and a program-0 diode 318-00. The MTJ 311-00 is coupled toa supply voltage V at one end, to the N terminal of program-1 diode317-00 and to the P terminal of program-0 diode 318-00 at the other end.The P terminal of the program-1 diode 317-00 is coupled to a supplyvoltage V+. The N terminal of the program-0 diode 318-00 is coupled toanother supply voltage V−. The other cells 310-01, 310-10, and 310-11are similarly coupled. The voltage Vs of the cells 310-00 and 310-10 inthe same columns are connected to BL0. The voltage Vs of the cells310-01 and 310-11 in the same column are connected to BL1. The voltagesV+ and V− of the cells 310-00 and 310-01 in the same row are connectedto WL0P and WL0N, respectively. The voltages V+ and V− of the cells310-10 and 310-11 in the same row are connected to WL1P and WL1N,respectively. To program a 0 into the cell 310-01, WL0N is set low andBL1 is set high, while setting the other BL and WLs at proper voltagesas shown in FIG. 13( a) to disable the other program-1 and program-0diodes. The bold line in FIG. 13( a) shows the direction of currentflow.

FIG. 13( b) shows alternative program-0 conditions for the cell 310-01in a 2×2 MRAM array in accordance with one embodiment. For example, toprogram a 0 into cell 310-01, set BL1 and WL0N to high and low,respectively. If BL0 is set to low in condition 1, the WL0P and WL1P canbe either low or floating, and WL1N can be either high or floating. Thehigh and low voltages of an MRAM in today's technologies are about 2-3Vfor high voltage and 0 for low voltage, respectively. If BL0 is floatingin condition 2, WL0P and WL1P can be high, low, or floating, and WL1Ncan be either high or floating. In a practical implementation, thefloating nodes are usually coupled to very weak devices to a fixedvoltage to prevent leakage. One embodiment of the program-0 condition isas shown in FIG. 13( a) without any nodes floating.

The cells in 2×2 MRAM arrays in FIGS. 12( a), 12(b), 13(a) and 13(b) arethree-terminal cells, namely, cells with V, V+, and V− nodes. However,if the program voltage VDDP is less than twice a diode's thresholdvoltage Vd, i.e. VDDP<2*Vd, the V+ and V− nodes of the same cell can beconnected together as a two-terminal cell. Since Vd is about 0.6-0.7V atroom temperature, this two-terminal cell works if the program highvoltage is less than 1.2V and low voltage is 0V. This is a commonvoltage configuration of MRAM arrays for advanced CMOS technologies thathas supply voltage of about 1.0V. FIGS. 14( a) and 14(b) show schematicsfor programming a 1 and 0, respectively, in a two-terminal 2×2 MRAMarray.

FIGS. 14( a) and 14(b) show one embodiment of programming 1 and 0,respectively, in a two-terminal 2×2 MRAM cell array in accordance withone embodiment. The cells 310-00, 310-01, 310-10, and 310-11 areorganized in a two-dimensional array. The cell 310-00 has the MTJ311-00, the program-1 diode 317-00, and the program-0 diode 318-00. TheMTJ 311-00 is coupled to a supply voltage V at one end, to the Nterminal of program-1 diode 317-00 and the P terminal of program-0 diode318-00 at the other end. The P terminal of the program-1 diode 317-00 iscoupled to a supply voltage V+. The N terminal of the program-0 diode318-00 is coupled to another supply voltage V−. The voltages V+ and V−are connected together in the cell level if VDDP<2*Vd can be met. Theother cells 310-01, 310-10 and 310-11 are similarly coupled. Thevoltages Vs of the cells 310-00 and 310-10 in the same columns areconnected to BL0. The voltage Vs of the cells 310-01 and 310-11 in thesame column are connected to BL1. The voltages V+ and V− of the cells310-00 and 310-01 in the same row are connected to WL0. The voltages V+and V− of the cells 310-10 and 310-11 in the same row are connected toWL1.

To program a 1 into the cell 310-01, WL0 is set high and BL1 is set low,while setting the other BL and WLs at proper voltages as shown in FIG.14( a) to disable other program-1 and program-0 diodes. The bold line inFIG. 14( a) shows the direction of current flow. To program a 0 into thecell 310-01, WL0 is set low and BL1 is set high, while setting the otherBL and WLs at proper voltages as shown in FIG. 14( b) to disable theother program-1 and program-0 diodes. The bold line in FIG. 14( b) showsthe direction of current flow.

The embodiments of constructing MRAM cells in a 2×2 array as shown inFIGS. 12( a)-14(b) are for illustrative purposes. Those skilled in theart understand that the number of cells, rows, or columns in a memorycan be constructed arbitrarily and rows and columns are interchangeable.

The programmable resistive devices can be used to construct a memory inaccordance with one embodiment. FIG. 15 shows a portion of aprogrammable resistive memory 100 constructed by an array 101 of n-rowby (m+1)-column non-MRAM cells 110 and n wordline drivers 150-i, wherei=0, 1, . . . , n−1, in accordance with one embodiment. The memory array101 has m normal columns and one reference column for one shared senseamplifier 140 for differential sensing. Each of the memory cells 110 hasa resistive element 111 coupled to the P terminal of a diode 112 asprogram selector and to a bitline BLj 170-j (j=0, 1, . . . m−1) orreference bitline BLR0 175-0 for those of the memory cells 110 in thesame column. The N terminal of the diode 112 is coupled to a wordlineWLBi 152-i through a local wordline LWLBi 154-i, where i=0, 1, . . . ,n−1, for those of the memory cells 110 in the same row. Each wordlineWLBi is coupled to at least one local wordline LWLBi, where i=0, 1, . .. , n−1. The LWLBi 154-i is generally constructed by a high resistivitymaterial, such as N well or polysilicon, to connect cells, and thencoupled to the WLBi (e.g., a low-resistivity metal WLBi) throughconductive contacts or vias, buffers, or post-decoders 172-i, where i=0,1, . . . , n−1. Buffers or post-decoders 172-i may be needed when usingdiodes as program selectors because there are currents flowing throughthe WLBi, especially when one WLBi drives multiple cells for program orread simultaneously in other embodiments. The wordline WLBi is driven bythe wordline driver 150-i with a supply voltage vddi that can beswitched between different voltages for program and read. Each BLj 170-jor BLR0 175-0 is coupled to a supply voltage VDDP through a Y-write passgate 120-j or 125 for programming, where each BLj 170-j or BLR0 175-0 isselected by YSWBj (j=0, 1, . . . , m−1) or YSWRB0, respectively. TheY-write pass gate 120-j (j=0, 1, . . . , m−1) or 125 can be built byPMOS, though NMOS, diode, or bipolar devices can be employed in someembodiments. Each BL or BLR0 is coupled to a dataline DL or DLR0 througha Y-read pass gate 130-j or 135 selected by YSRj (j=0, 1, . . . , m−1)or YSRR0, respectively. In this portion of memory array 101, m normaldatalines DLj (j=0, 1, . . . , m−1) are connected to an input 160 of asense amplifier 140. The reference dataline DLR0 provides another input161 for the sense amplifier 140 (no multiplex is generally needed in thereference branch). The output of the sense amplifiers 140 is Q0.

To program a cell, the specific WLBi and YSWBj are turned on and a highvoltage is supplied to VDDP, where i=0, 1, . . . n−1 and j=0, 1, . . . ,m−1. In some embodiments, the reference cells can be programmed to 0 or1 by turning on WLRBi, and YSWRB0, where i=0, 1, . . . , n−1. To read acell, a data column 160 can be selected by turning on the specific WLBiand YSRj, where i=0, 1, . . . , n−1, and j=0, 1, . . . , m−1, and areference cell coupled to the reference dataline DLR0 161 for the senseamplifier 140 can be selected to sense and compare the resistancedifference between BLs and ground, while disabling all YSWBj and YSWRB0where j=0, 1, . . . , m−1.

The programmable resistive devices can be used to construct a memory inaccordance with one embodiment. FIG. 16( a) shows a portion of aprogrammable resistive memory 100 constructed by an array 101 of3-terminal MRAM cells 110 in n rows and m+1 columns and n pairs ofwordline drivers 150-i and 151-i, where i=0, 1, . . . , n−1, accordingto one embodiment. The memory array 101 has m normal columns and onereference column for one shared sense amplifier 140 for differentialsensing. Each of the memory cells 110 has a resistive element 111coupled to the P terminal of a program-0 diode 112 and N terminal of aprogram-1 diode 113. The program-0 diode 112 and the program-1 diode 113serve as program selectors. Each resistive element 111 is also coupledto a bitline BLj 1701 (j=0, 1, . . . m−1) or reference bitline BLR0175-0 for those of the memory cells 110 in the same column. The Nterminal of the diode 112 is coupled to a wordline WLNi 152-i through alocal wordline LWLNi 154-i, where i=0, 1, . . . , n−1, for those of thememory cells 110 in the same row. The P terminal of the diode 113 iscoupled to a wordline WLPi 153-i through a local wordline LWLPi 155-i,where i=0, 1, . . . , n−1, for those cells in the same row. Eachwordline WLNi or WLPi is coupled to at least one local wordline LWLNi orLWLPi, respectively, where i=0, 1, . . . , n−1. The LWLNi 154-i andLWLPi 155-i are generally constructed by a high resistivity material,such as N well or polysilicon, to connect cells, and then coupled to theWLNi or WLPi (e.g., low-resistivity metal WLNi or WLPi) throughconductive contacts or vias, buffers, or post-decoders 172-i or 173-irespectively, where i=0, 1, . . . , n−1. Buffers or post-decoders 172-ior 173-i may be needed when using diodes as program selectors becausethere are currents flowing through WLNi or WLPi, especially when oneWLNi or WLPi drivers multiple cells for program or read simultaneouslyin some embodiments. The wordlines WLNi and WLPi are driven by wordlinedrivers 150-i and 151-i, respectively, with a supply voltage vddi thatcan be switched between different voltages for program and read. EachBLj 170-j or BLR0 175-0 is coupled to a supply voltage VDDP through aY-write-0 pass gate 120-0 or 125 to program 0, where each BLj 1701 orBLR0 175-0 is selected by YS0WBj (j=0, 1, . . . , m−1) or YS0WRB0,respectively. Y-write-0 pass gate 120-j or 125 can be built by PMOS,though NMOS, diode, or bipolar devices can be employed in otherembodiments. Similarly, each BLj 170-j or BLR0 175-0 is coupled to asupply voltage 0V through a Y-write-1 pass gate 121-j or 126 to program1, where each BLj 170-j or BLR0 175-0 is selected by YS1Wj (j=0, 1, . .. , m−1) or YS1WR0, respectively. Y-write-1 pass gate 121-j or 126 iscan be built by NMOS, though PMOS, diode, or bipolar devices can beemployed in other embodiments. Each BL or BLR0 is coupled to a datalineDL or DLR0 through a Y-read pass gate 130-j or 135 selected by YSRj(j=0, 1, . . . , m−1) or YSRR0, respectively. In this portion of memoryarray 101, m normal datalines DLj (j=0, 1, . . . , m−1) are connected toan input 160 of a sense amplifier 140. Reference dataline DLR0 providesanother input 161 for the sense amplifier 140, except that no multiplexis generally needed in a reference branch. The output of the senseamplifier 140 is Q0.

To program a 0 into a cell, the specific WLNi, WLPi and BLj are selectedas shown in FIG. 13( a) or 13(b) by wordline drivers 150-i, 151-i, andY-pass gate 120-j by YS0WBj, respectively, where i=0, 1, . . . n−1 andj=0, 1, . . . , m−1, while the other wordlines and bitlines are alsoproperly set. A high voltage is applied to VDDP. In some embodiments,the reference cells can be programmed into 0 by setting proper voltagesto WLRNi 158-i, WLRPi 159-i and YS0WRB0, where i=0, 1, . . . , n−1. Toprogram a 1 to a cell, the specific WLNi, WLPi and BLj are selected asshown in FIG. 12( a) or 12(b) by wordline driver 150-i, 151-i, andY-pass gate 121-j by YS1Wj, respectively, where i=0, 1, . . . n−1 andj=0, 1, . . . , m−1, while the other wordlines and bitlines are alsoproperly set. In some embodiments, the reference cells can be programmedto 1 by setting proper voltages to WLRNi 158-i, WLRPi 159-i and YS1WR0,where i=0, 1, . . . , n−1. To read a cell, a data column 160 can beselected by turning on the specific WLNi, WLPi and YSRj, where i=0, 1, .. . , n−1, and j=0, 1, . . . , m−1, and a reference cell coupled to thereference dataline DLR 161 for the sense amplifier 140 to sense andcompare the resistance difference between BLs and ground, whiledisabling all YS0WBj, YS0WRB0, YS1Wj and YS1WR0, where j=0, 1, . . . ,m−1.

Another embodiment of constructing an MRAM memory with 2-terminal MRAMcells is shown in FIG. 16( b), provided the voltage difference VDDP,between high and low states, is less than twice of the diode's thresholdvoltage Vd, i.e., VDDP<2*Vd. As shown in FIG. 16( b), two wordlines perrow WLNi 152-i and WLPi 153-i in FIG. 16( a) can be merged into onewordline driver WLNi 152-i, where i=0, 1, . . . , n−1. Also, the localwordlines LWLNi 154-i and LWLP 155-i per row in FIG. 16( a) can bemerged into one local wordline LWLNi 154-i, where i=0, 1, . . . , n−1,as shown in FIG. 16( b). Still further, two wordline drivers 150-i and151-i in FIG. 16( a) can be merged into one, i.e., wordline driver150-i. The BLs and WLNs of the unselected cells are applied with properprogram 1 and 0 conditions as shown in FIGS. 14( a) and 14(b),respectively. Since half of wordlines, local wordlines, and wordlinedrivers can be eliminated in this embodiment, cell and macro areas canbe reduced substantially.

Differential sensing is a common for programmable resistive memory,though single-end sensing can be used in other embodiments. FIGS. 17(a), 17(b), and 17(c) show three other embodiments of constructingreference cells for differential sensing. In FIG. 17( a), a portion ofmemory 400 has a normal array 180 of n×m cells, two reference columns150-0 and 150-1 of n×1 cells each storing all data 0 and 1 respectively,m+1 Y-read pass gates 130, and a sense amplifier 140. As an example, n=8and m=8 are used to illustrate the concept. There are n wordlines WLBiand n reference wordlines WLRBi for each row, where i=0, 1, . . . , n−1.When a wordline WLBi is turned on to access a row, a correspondingreference wordline WLRBi (i=0, 1, . . . , n−1) is also turned on toactivate two reference cells 170-0 and 170-1 in the same row to providemid-level resistance after proper scaling in the sense amplifier. Theselected dataline 160 along with the reference dataline 161 are input toa sense amplifier 140 to generate an output Q0. In this embodiment, eachWLRBi and WLBi (i=0, 1, . . . , n−1) are hardwired together and everycells in the reference columns need to be pre-programmed before read.

FIG. 17( b) shows another embodiment of using a reference cell externalto a reference column. In FIG. 17( b), a portion of memory 400 has anormal array 180 of n×m cells, a reference column 150 of n×1 cells, m+1Y-read pass gates 130, and a sense amplifier 140. When a wordline WLBi(i=0, 1, . . . , n−1) is turned on, none of the cells in the referencecolumn 150 are turned on. An external reference cell 170 with apre-determined resistance is turned on instead by an external referencewordline WLRB. The selected dataline 160 and the reference dataline 161are input to a sense amplifier 140 to generate an output Q0. In thisembodiment, all internal reference wordlines WLRBi (i=0, 1, . . . , n−1)in each row are tied together to a high voltage to disable the diodes inthe reference column. The reference column 150 provides a loading tomatch with that of the normal columns.

FIG. 17( c) shows another embodiment of constructing reference cells fordifferential sensing. In FIG. 17( c), a portion of memory 400 has anormal array 180 of n×m cells, one reference column 150 of n×1, tworeference rows 175-0 and 175-1 of 1×m cells, m+1 Y-read pass gates 130,and a sense amplifier 140. As an example, n=8 and m=8 are used toillustrate the approach. There are n wordlines WLBi and 2 referencewordlines WLRB0 175-0 and WLRB1 175-1 on top and bottom of the array,where i=0, 1, . . . , n−1. When a wordline WLBi (i=0, 1, . . . , n−1) isturned on to access a row, the reference wordline WLRB0 and WLRB1 arealso turned on to activate two reference cells 170-0 and 170-1 in theupper and lower right corners of the array 180, which store data 0 and 1respectively. The selected dataline 160 along with the referencedataline 161 are input to a sense amplifier 140 to generate an outputQ0. In this embodiment, all cells in the reference column 150 aredisabled except that the cells 170-0 and 170-1 on top and bottom of thereference column 150. Only two reference cells are used for the entiren×m array that needs to be pre-programmed before read.

For those programmable resistive devices that have a very smallresistance ratio between states 1 and 0, such as 2:1 ratio in MRAM,FIGS. 17( a) and 17(c) are desirable embodiments, depending on how manycells are suitable for one pair of reference cells. Otherwise, FIG. 17(b) is a desirable embodiment for electrical fuse or PCM that hasresistance ratio of more than about 10.

FIGS. 15, 16(a), 16(b), 17(a), 17(b), and 17(c) show only a fewembodiments of a portion of programmable resistive memory in asimplified manner. The memory array 101 in FIGS. 15, 16(a), and 16(b)can be replicated s times to read or program s-cells at the same time.In the case of differential sensing, the number of reference columns tonormal columns may vary and the physical location can also vary relativeto the normal data columns. Rows and columns are interchangeable. Thenumbers of rows, columns, or cells likewise may vary. For those skilledin the art understand that the above descriptions are for illustrativepurpose. Various embodiments of array structures, configurations, andcircuits are possible and are still within the scope of this invention.

The portions of programmable resistive memories shown in FIGS. 15,16(a), 16(b), 17(a), 17(b) and 17(c) can include different types ofresistive elements. The resistive element can be an electrical fuseincluding a fuse fabricated from an interconnect, contact/via fuse,contact/via anti-fuse, or gate oxide breakdown anti-fuse. Theinterconnect fuse can be formed from silicide, metal, metal alloy, orsome combination thereof, or can be constructed from a CMOS gate. Theresistive element can also be fabricated from phase-change material,MTJ, etc. For the electrical fuse fabricated from an interconnect,contact, or via fuse, programming requirement is to provide asufficiently high current, about 4-20 mA range, for a few microsecondsto blow the fuse by electro-migration, heat, ion diffusion, or somecombination thereof. For anti-fuse, programming requirement is toprovide a sufficiently high voltage to breakdown the dielectrics betweentwo ends of a contact, via or CMOS gate. The required voltage is about6-7V for a few millisecond to consume about 10 uA of current in today'stechnologies. Programming Phase-Change Memory (PCM) requires differentvoltages and durations for 0 and 1. Programming to a 1 (or to reset)requires a high and short voltage pulse applied to the phase-changefilm. Alternatively, programming to a 0 (or to set) requires a low andlong voltage pulse applied to the phase change film. The reset needsabout 3V for 50 ns and consumes about 300 uA, while set needs about 2Vfor 300 ns and consumes about 100 uA. For MRAM, the high and low programvoltages are about 2-3V and 0V, respectively, and the current is about+/−100-200 uA.

Most programmable resistive devices have a higher voltage VDDP (˜2-3V)for programming than the core logic supply voltage VDD (˜1.0V) forreading. FIG. 18( a) shows a schematic of a wordline driver circuit 60according to one embodiment. The wordline driver includes devices 62 and61, as shown as the wordline driver 150 in FIGS. 15, 16(a) and 16(b).The supply voltage vddi is further coupled to either VDDP or VDD throughpower selectors 63 and 64 (e.g., PMOS power selectors) respectively. Theinput of the wordline driver Vin is from an output of an X-decoder. Insome embodiments, the power selectors 63 and 64 are implemented as thickoxide I/O devices to sustain high voltage. The bodies of power selector63 and 64 can be tied to vddi to prevent latchup.

Similarly, bitlines tend to have a higher voltage VDDP (˜2-3V) forprogramming than the core logic supply voltage VDD (˜1.0V) for reading.FIG. 18( b) shows a schematic of a bitline circuit 70 according to oneembodiment. The bitline circuit 70 includes a bitline (BL) coupled toVDDP and VDD through power selectors 73 and 74 (e.g., PMOS powerselectors), respectively. If the bitline needs to sink a current such asin an MRAM, an NMOS pulldown device 71 can be provided. In someembodiments, the power selectors 73 and 74 as well as the pulldowndevice 71 can be implemented as thick-oxide I/O devices to sustain highvoltage. The bodies of power selector 73 and 74 can be tied to vddi toprevent latchup.

Using junction diodes as program selectors may have high leakage currentif a memory size is very large. Power selectors for a memory can helpreducing leakage current by switching to a lower supply voltage or eventurning off when a portion of memory is not in use. FIG. 18( c) shows aportion of memory 85 with an internal power supply VDDP coupled to anexternal supply VDDPP and a core logic supply VDD through powerselectors 83 and 84. VDDP can even be coupled to ground by an NMOSpulldown device 81 to disable this portion of memory 85, if this portionof memory is temporarily not in use.

FIGS. 19( a) and 20(a) only show two of many pre-amplifier embodiments.Similarly, FIGS. 19( b), 20(b) and 20(c) only show several of manyamplifier and level shifter embodiments. Various combinations ofpre-amplifiers, level shifters, and amplifiers in core logic or I/Odevices can be constructed differently, separately or mixed.

FIG. 19( a) shows one embodiment of a schematic of a pre-amplifier 100according to one embodiment. The pre-amplifier 100 needs specialconsiderations because the supply voltage VDD for core logic devices isabout 1.0V that does not have enough head room to turn on a diode tomake sense amplifiers functional, considering a diode's threshold isabout 0.7V. One embodiment is to use another supply VDDR, higher thanVDD, to power at least the first stage of sense amplifiers. Theprogrammable resistive cell 110 shown in FIG. 19( a) has a resistiveelement 111 and a diode 112 as program selector, and can be selected forread by asserting YSR′ to turn on a gate of a NMOS 130 (NMOS device) andwordline bar WLB. The pre-amplifier 100 also has a reference cell 115including a reference resistive element 116 and a reference diode 117.The reference cell 115 can be selected for differential sensing byasserting YSR′ to turn on a gate of a NMOS 131 and reference wordlineWLRB. The resistance Ref of the reference resistive element 116 can beset at a resistance half-way between minimum of state 1 and maximum ofstate 0 resistance.

The drains of NMOS 130 and 131 are coupled to sources of NMOS 132 and134, respectively. The gates of 132 and 134 are biased at a fixedvoltage Vbias. The channel width to length ratios of NMOS 132 and 134can be relatively large to clamp the voltage swings of bitline BL andreference bitline BLR, respectively. The drain of NMOS 132 and 134 arecoupled to drains of PMOS 170 and 171, respectively. The drain of PMOS170 is coupled to the gate of PMOS 171 and the drain of PMOS 171 iscoupled to the gate of PMOS 170. The outputs V+ and V− of thepre-amplifier 100 are drains of PMOS 170 and PMOS 171 respectively. Thesources of PMOS 170 and PMOS 171 are coupled to a read supply voltageVDDR. The outputs V+ and V− are pulled up by a pair of PMOS 175 to VDDRwhen the pre-amplifier 100 is disabled. VDDR is about 2-3V (which ishigher than about 1.0V VDD of core logic devices) to turn on the diodeselectors 112 and 117 in the programmable resistive cell 110 and thereference cell 115, respectively. The CMOS 130, 131, 132, 134, 170, 171,and 175 can be embodied as thick-oxide I/O devices to sustain highvoltage VDDR. In another embodiment, the read selectors 130 and 131 canbe PMOS devices.

FIG. 19( b) shows one embodiment of a schematic of an amplifier 200according to one embodiment. In another embodiment, the outputs V+ andV− of the pre-amplifier 100 in FIG. 19( a) can be coupled to gates ofNMOS 234 and 232, respectively, of the amplifier 200. The NMOS 234 and232 can be relatively thick oxide I/O devices to sustain the high inputvoltage V+ and V− from a pre-amplifier. The sources of NMOS 234 and 232are coupled to drains of NMOS 231 and 230, respectively. The sources ofNMOS 231 and 230 are coupled to a drain of an NMOS 211. The gate of NMOS211 is coupled to a clock φ to turn on the amplifier 200, while thesource of NMOS 211 is coupled to ground. The drains of NMOS 234 and 232are coupled to drains of PMOS 271 and 270, respectively. The sources ofPMOS 271 and 270 are coupled to a core logic supply VDD. The gates ofPMOS 271 and NMOS 231 are connected and coupled to the drain of PMOS270, as a node Vp. Similarly, the gates of PMOS 270 and NMOS 230 areconnected and coupled to the drain of PMOS 271, as a node Vn. The nodesVp and Vn are pulled up by a pair of PMOS 275 to VDD when the amplifier200 is disabled when φ goes low. The output nodes Vout+ and Vout− arecoupled to nodes Vn and Vp through a pair of inverters as buffers.

FIG. 19( c) shows a timing diagram of the pre-amplifier 100 and theamplifier 200 in FIGS. 19( a) and 19(b), respectively. The X- andY-addresses AX/AY are selected to read a cell. After some propagationdelays, a cell is selected for read by turning WLB low and YSR high tothereby select a row and a column, respectively. Before activating thepre-amplifier 100, a pulse Vpc is generated to precharge DL and DLR toground. The pre-amplifier 100 would be very slow if the DL and DLRvoltages are high enough to turn off the cascode devices (e.g., NMOS 132and 134). After the pre-amplifier outputs V+ and V− are stabilized, theclock φ is set high to turn on the amplifier 200 and to amplify thefinal output Vout+ and Vout− into full logic levels.

FIG. 20( a) shows another embodiment of a pre-amplifier 100′, similar tothe pre-amplifier 100 in FIG. 18( a). The reference branch is turned onby a level signal to enable a sense amplifier, SAEN, rather than cycleby cycle in FIG. 19( a). The PMOS pull-ups 171 and 170 in FIG. 20( a)are configured as current mirror loads, rather than a pair ofcross-coupled PMOS in FIG. 19( a). In this embodiment, the number of thereference branches can be shared at the expense of increasing powerconsumption.

FIG. 20( b) shows level shifters 300 according to one embodiment. The V+and V− from the pre-amplifier 100, 100′ outputs in FIG. 19( a) or FIG.20( a) are coupled to gates of NMOS 301 and 302, respectively. Thedrains of NMOS 301 and 302 are coupled to a supply voltage VDDR. Thesources of NMOS 301 and 302 are coupled to drains of NMOS 303 and 304,respectively, which have gates and drains connected as diodes to shiftthe voltage level down by one Vtn, the threshold voltage of an NMOS. Thesources of NMOS 303 and 304 are coupled to pulldown devices NMOS 305 and306, respectively. The gates of NMOS 305 and 306 can be turned on by aclock φ. The NMOS 301, 302, 303 and 304 can be thick-oxide I/O devicesto sustain high voltage VDDR. The NMOS 303 and 304 can be cascaded morethan once to shift V+ and V− further to proper voltage levels Vp and Vn.In another embodiment, the level shifting devices 303 and 304 can bebuilt using PMOS devices.

FIG. 20( c) shows another embodiment of an amplifier 200′ withcurrent-mirror loads having PMOS 270 and 271. The inputs Vp and Vn ofthe amplifier 200′ are from the outputs Vp and Vn of the level shifter300 in FIG. 20( b) can be coupled to gates of NMOS 231 and 230,respectively. The drains of NMOS 231 and 230 are coupled to drains ofNMOS 271 and 270 which provide current-mirror loads. The drain and gateof PMOS 271 are connected and coupled to the gate of PMOS 270. Thesources of NMOS 231 and 230 are coupled to the drain of an NMOS 211,which has the gate coupled to a clock signal φ and the source to ground.The clock signal φ enables the amplifier 200. The drain of PMOS 270provides an output Vout+. The PMOS pullup 275 keeps the output Vout+ atlogic high level when the amplifier 200′ is disabled.

FIGS. 21( a) and 21(b) show a flow chart depicting embodiments of aprogram method 700 and a read method 800, respectively, for aprogrammable resistive memory in accordance with certain embodiments.The methods 700 and 800 are described in the context a programmableresistive memory, such as the programmable resistive memory 100 in FIGS.15, 16(a) and 16(c). In addition, although described as a flow of steps,one of ordinary skilled in the art will recognize that at least some ofthe steps may be performed in a different order, includingsimultaneously, or skipped.

FIG. 21( a) depicts a method 700 of programming a programmable resistivememory in a flow chart according to one embodiment. In the first step710, proper power selectors can be selected so that high voltages can beapplied to the power supplies of wordline drivers and bitlines. In thesecond step 720, the data to be programmed in a control logic (not shownin FIGS. 15, 16(a), and 16(b)) can be analyzed, depending on what typesof programmable resistive devices. For electrical fuse, this is aOne-Time-Programmable (OTP) device such that programming always meansblowing fuses into a non-virgin state and is irreversible. Programvoltage and duration tend to be determined by external control signals,rather than generated internally from the memory. For PCM, programminginto a 1 (to reset) and programming into a 0 (to set) require differentvoltages and durations such that a control logic determines the inputdata and select proper power selectors and assert control signals withproper timings. For MRAM, the directions of current flowing through MTJsare more important than time duration. A control logic determines properpower selectors for wordlines and bitlines and assert control signals toensure a current flowing in the desired direction for desired time. Inthe third step 730, a cell in a row can be selected and thecorresponding local wordline can be turned on. In the fourth step 740,sense amplifiers can be disabled to save power and prevent interferencewith the program operations. In the fifth step 750, a cell in a columncan be selected and the corresponding Y-write pass gate can be turned onto couple the selected bitline to a supply voltage. In the last step760, a desired current can be driven for a desired time in anestablished conduction path to complete the program operations. For mostprogrammable resistive memories, this conduction path is from a highvoltage supply through a bitline select, resistive element, diode asprogram selector, and an NMOS pulldown of a local wordline driver toground. Particularly, for programming a 1 to an MRAM, the conductionpath is from a high voltage supply through a PMOS pullup of a localwordline driver, diode as program selector, resistive element, andbitline select to ground.

FIG. 21( b) depicts a method 800 of reading a programmable resistivememory in a flow chart according to one embodiment. In the first step810, proper power selectors can be selected to provide supply voltagesfor local wordline drivers, sense amplifiers, and other circuits. In thesecond step 820, all Y-write pass gates, i.e. bitline program selectors,can be disabled. In the third step 830, desired local wordline(s) can beselected so that the diode(s) as program selector(s) have a conductionpath to ground. In the fourth step 840, sense amplifiers can be enabledand prepared for sensing incoming signals. In the fifth step 850, thedataline and the reference dataline can be pre-charged to the V− voltageof the programmable resistive device cell. In the sixth step 860, thedesired Y-read pass gate can be selected so that the desired bitline iscoupled to an input of the sense amplifier. A conduction path is thusestablished from the bitline to the resistive element in the desiredcell, diode(s) as program selector(s), and the pulldown of the localwordline driver(s) to ground. The same applies for the reference branch.In the last step 870, the sense amplifiers can compare the read currentwith the reference current to determine a logic output of 0 or 1 tocomplete the read operations.

FIG. 22 shows a processor system 700 according to one embodiment. Theprocessor system 700 can include a programmable resistive device 744,such as in a cell array 742, in memory 740, according to one embodiment.The processor system 700 can, for example, pertain to a computer system.The computer system can include a Central Process Unit (CPU) 710, whichcommunicate through a common bus 715 to various memory and peripheraldevices such as I/O 720, hard disk drive 730, CDROM 750, memory 740, andother memory 760. Other memory 760 is a conventional memory such asSRAM, DRAM, or flash, typically interfaces to CPU 710 through a memorycontroller. CPU 710 generally is a microprocessor, a digital signalprocessor, or other programmable digital logic devices. Memory 740 ispreferably constructed as an integrated circuit, which includes thememory array 742 having at least one programmable resistive device 744.The memory 740 typically interfaces to CPU 710 through a memorycontroller. If desired, the memory 740 may be combined with theprocessor, for example CPU 710, in a single integrated circuit.

The invention can be implemented in a part or all of an integratedcircuit in a Printed Circuit Board (PCB), or in a system. Theprogrammable resistive device can be fuse, anti-fuse, or emergingnonvolatile memory. The fuse can be silicided or non-silicidedpolysilicon fuse, metal fuse, contact fuse, or via fuse. The anti-fusecan be a gate-oxide breakdown anti-fuse, contact or via anti-fuse withdielectrics in-between. The emerging nonvolatile memory can be MagneticRAM (MRAM), Phase Change Memory (PCM), Conductive Bridge RAM (CBRAM), orResistive RAM (RRAM). Though the program mechanisms are different, theirlogic states can be distinguished by different resistance values.

The above description and drawings are only to be consideredillustrative of exemplary embodiments, which achieve the features andadvantages of the present invention. Modifications and substitutions ofspecific process conditions and structures can be made without departingfrom the spirit and scope of the present invention.

The many features and advantages of the present invention are apparentfrom the written description and, thus, it is intended by the appendedclaims to cover all such features and advantages of the invention.Further, since numerous modifications and changes will readily occur tothose skilled in the art, it is not desired to limit the invention tothe exact construction and operation as illustrated and described.Hence, all suitable modifications and equivalents may be resorted to asfalling within the scope of the invention.

What is claimed is:
 1. A reversible resistive memory, comprising: aplurality of reversible resistive cells, each of the reversibleresistive cells including a reversible resistive film and a diode asprogram selector having at least a first active region with a first typeof dopant for a first terminal of the diode, and a second active regionwith a second type of dopant for a second terminal of the diode, bothactive regions being fabricated from sources or drains of CMOS devicesand residing in a common CMOS well, the first terminal of the diodebeing coupled to a first terminal of the reversible resistive film, thereversible resistive film being programmable reversibly by conducting acurrent flowing through the reversible resistive film, wherein thecurrent is high and/or short duration for programming one state and lowand/or long duration for programming another state; a plurality of localwordlines, each coupled to a plurality of the reversible resistive cellsvia the second terminal of the diodes and having a first resistivity; aplurality of global wordlines, each coupled to at least one of the localwordlines and having a second resistivity; and a plurality of bitlines,each coupled to a plurality of the reversible resistive cells via thesecond terminal of reversible resistive film.
 2. A reversible resistivememory as recited in claim 1, wherein the reversible resistive film is aphase-change film that has chemical composition of Germanium (Ge),Antimony (Sb), and Tellurium (Te).
 3. A reversible resistive memory asrecited in claim 2, wherein the phase-change film has the chemicalcomposition of Germanium (Ge), Antimony (Sb), and Tellurium (Te) asGexSbyTez, where x+y+z=9, and x=2+/−10%, y=2+/−10%, and z=5+−10%.
 4. Areversible resistive memory as recited in claim 1, wherein thereversible resistive film comprises a metal oxide between metal or metalalloy electrodes.
 5. A reversible resistive memory as recited in claim1, wherein the reversible resistive film comprises a solid-stateelectrolyte between metal or metal alloy electrodes.
 6. A reversibleresistive memory as recited in claim 1, wherein at least one of thelocal wordlines is constructed at least in part of CMOS well.
 7. Areversible resistive memory as recited in claim 1, wherein at least oneof the global wordlines is constructed of metal.
 8. A reversibleresistive memory as recited in claim 1, wherein at least one of theglobal wordlines is coupled to at least one of the local wordlinesthrough at least one of conductive via or contact.
 9. A reversibleresistive memory as recited in claim 1, wherein at least one of thebitlines is coupled to a first supply voltage line and at least one ofthe global wordlines is coupled to a second supply voltage line duringprogram or read, wherein the voltage between the first and second supplyvoltages has larger magnitude or longer duration for program than read.10. A reversible resistive memory as recited in claim 1, wherein atleast one of the bitlines is coupled to a first supply voltage line andat least one of the global wordlines is coupled to a second supplyvoltage line during program, and wherein the voltage between the firstand second supply voltage lines has larger magnitude or longer durationfor programming into the one state than the another state.
 11. Areversible resistive memory as recited in claim 1, wherein at least oneof the bitlines is coupled to a first supply voltage line and at leastone of the global wordlines is coupled to a second supply voltage lineduring program, and wherein the voltage between the first and secondsupply voltage lines has voltage-limit or current-limit to program intoone state.
 12. A reversible resistive memory as recited in claim 1,wherein the reversible resistive memory further comprising a pluralityof write bitline selectors to select at least one of the bitlinescoupled to a first supply voltage line, and a plurality of wordlinedrivers to select at least one of local wordlines coupled to a secondsupply voltage line to conduct a current during program.
 13. Areversible resistive memory as recited in claim 1, wherein thereversible resistive memory further comprises a plurality of readbitline selectors to select at least one of the bitlines coupled to asense amplifier, and a plurality of wordline drivers to select at leastone of local wordlines coupled to a supply voltage line to conduct acurrent through the sense amplifier during read.
 14. An electronicssystem, comprising: a processor; and a reversible resistive memoryoperatively connected to the processor, the reversible resistive memoryincluding at least: a plurality of reversible resistive cells, eachincluding a reversible resistive film and a diode as program selectorhaving at least a first active region with a first type of dopant for afirst terminal of the diode, and a second active region with a secondtype of dopant for a second terminal of the diode, both active regionsbeing fabricated from sources or drains of CMOS devices and residing ina common CMOS well, the first terminal of the diode being coupled to afirst terminal of the reversible resistive film, the reversibleresistive film being programmable by conducting a current flowingthrough the reversible resistive film and the program selector, whereinthe current is high and/or short duration for one state and low and/orlong duration for another state; a plurality of local wordlines, eachcoupled to a plurality of the reversible resistive cells via the secondterminal of the diodes and having a first resistivity; a plurality ofglobal wordlines, each coupled to at least one of the local wordlinesand having a second resistivity; and a plurality of bitlines, eachcoupled to a plurality of the reversible resistive cells via the secondterminal of reversible resistive film.
 15. An electronics system asrecited in claim 14, wherein the reversible resistive film is aphase-change film that has chemical composition of Germanium (Ge),Antimony (Sb), and Tellurium (Te).
 16. An electronics system as recitedin claim 14, wherein the reversible resistive film comprises a metaloxide between metal or metal alloy electrodes.
 17. An electronics systemas recited in claim 14, wherein the reversible resistive film comprisesa solid-state electrolyte between metal or metal alloy electrodes. 18.An electronics system as recited in claim 14, wherein at least one ofthe local wordlines is constructed at least in part of CMOS well.
 19. Anelectronics system as recited in claim 14, wherein at least one of theglobal wordlines is constructed of metal.
 20. An electronics system asrecited in claim 14, wherein the electronics system is configured toperiodically read content of every cell and writes the content back. 21.A method for programming a reversible resistive memory, comprising:providing a plurality of reversible resistive cells, each including anreversible resistive film and a diode, the diode serving as a programselector and having at least a first active region with a first type ofdopant for a first terminal of the diode, and having a second activeregion with a second type of dopant for a second terminal of the diode,both active regions being fabricated from sources or drains of CMOSdevices and residing in a common CMOS well, the first terminal of thediode being coupled to a first terminal of the reversible resistivefilm, the reversible resistive film being programmable reversibly byconducting a current flowing through the reversible resistive film;providing a plurality of local wordlines, each coupled to a plurality ofthe reversible resistive cells via the second terminal of the diodes andhaving a first resistivity; providing a plurality of global wordlines,each coupled to at least one of the local wordlines and having a secondresistivity; providing a plurality of bitlines, each coupled to aplurality of the reversible resistive cells via a second terminal ofreversible resistive film; and programming at least one of thereversible resistive cells by applying voltages to a selected one of theglobal wordlines and to a selected one of the bitlines, and generating ahigh and/or short-duration current to reversibly program into one stateand a low and/or long-duration current to reversibly program intoanother state.